I’ll answer all here. Thanks for your advice.

Some sort of cargo fitting on the rack, like a backpack, is an idea. I wonder if the high location of the weight would be a problem?

The OEM front racks I’ve seen for Twenty, Shopper etc are wide, intended to support a shopping bag or packages, not for fitting panniers.

I suppose P-clamps would work, though I’ve only used them at rack tops, where they aren’t weight bearing. I think my idea of axle mounting will be sturdier. I pulled the trigger on the TopIt rack. I’ll post a pic once I work up the mounting.